Electronic fuel control systems are widely used in internal combustion engines to precisely meter the amount of fuel required for varying engine requirements. Such systems control the amount of fuel delivered for combustion in response to multiple system inputs including throttle angle and the concentration of oxygen in the exhaust gas produced by combustion of air and fuel. Typical electronic fuel control systems operate in a closed-loop mode in response to sensed exhaust oxygen level in order to maintain the ratio of air and fuel at or near stoichiometry. Improved forms of fuel control systems further include adaptive mechanisms which learn and remember the probable amount of fuel that needs to be injected under previously experienced engine operating conditions as specified by such sensed variables as engine speed, engine load, engine temperature, and fuel type.
When fuel control systems are designed to supply compressed gases such as natural gas, rather than gasoline and other liquid fuels, the system must also be sensitive to factors which are less consequential in liquid-fueled engines. While liquid-fuel pressure regulators are normally capable of delivering fuel at a substantially constant pressure (typically about 40 p.s.i.g.) over a wide range of operating conditions, the pressure regulators used with compressed natural gas supplied at higher pressure, typically around 100 p.s.i.g., cannot maintain constant pressure over the wide range of flow rates to which the engine is subjected. As pulses of fuel are released through the fuel injectors, pressure perturbations are induced in the fuel supply rail which are a function of numerous factors, such as engine load (typically calculated from measured air mass flow rate), engine speed, variations in the injector pulse width, regulator set point pressure, regulator and injector temperature, fuel supply tank pressure, and the design of the pressure regulator and the fuel supply rail. These pressure perturbations, measured to be as high as 6 psi, interact with the components of the fuel system to produce instabilities which cause the air/fuel mixture to deviate from its optimum level, adversely impacting engine emission levels.